Raman spectrophotometer



June 14, 1960 H. H. CARY RAMAN sPEcTRoPHoToMETER Filed March 2, 1956 5 Sheets-Sheet 1 ,4 TroR/VE Y June 14, 1960 H. H. CARY RAMAN sPEcTRoPHoToMETER 5 Sheets-Sheet 2 Filed March 2, 1956 INVENToR. HENRY CARY rroR/vgL-Y June 14, 1960 H. H. CARY RAMAN sPEcTRoPHoToMETER 5 Sheets-Sheet 5 Filed March 2, 1956- INVENTR.

HENRY H. CAR Y Arromvsy June 14, 1960 H. H. CARY 2,940,355

RAMAN SPECTROPHOTOMETER Filed March 2, 1956 5 Sheets-Sheet 4 INVENTOR.

H5N Y H. CARY ATToRA/EY June 14, 1960 H. H. CARY 2,940,355

RAMAN SPECTROPHOTOMETER Filed March 2, 1956 5 Sheets-Sheet 5 INVENTOR. /ENRY/CARY rToRA/Fy nited States Parent n srEcrRoPHoror/IETER `ltllenr'y H. Cary, Alhambra, Calif., assignor to Applied Physics Corporation, Monrovia, Calif., a corporation of California i Filed Mar. 2, 1956, Ser. No. 569,177

14 Claims. (Cl. 882-14) ,This invention relates to impr'vementsin spectroscopy and more particularly to improyements in Raritan specirescopy Numerous spectroscopic systerhsare employed in the analysis of various substances, Whether they be atorriic or molecular and whether they be in the form of gas', liquid, or solid. Raman spectromete'rs have found particular use in the study of molecular liquids. Raman spectroscopy is often more effective in the determination of molecularstructure and in the analysis of molecular 'mix'- tures than other spectroscopic techniques, particularly because the lines of a Raman spectrum are comparatively sharp and also because the number of lines 'present in Raman spectrum is often lower than the number of lines occurring in other types of spectra that might be employed lin the analysis of molecular compositions and also because the band of frequencies of a Raman spectrum lies within a narrower octave range. i

Raman spectra occur when light is scattered by matter. At the same time, scattering of other types also usually occurs. Radiation due to such other scattering tends to obscure or mask the Raman spectra. that the scattering process whereby Raman spectra are produced is so inefficient, and also due to the fact that other types of scattering occur when Raman spetra are produced, the detection and analysis of Raman spectra is very diiiicult.

To appreciate this difficulty, consider now the scattering process in more detail and assume for example'that it is desirable to detect the Raman spectrum of aliquid sample contained in a glass cell. In lthe production of Raman spectra, 'a source of `substantially monochromatic `exciting radiation is generally employed. n

Scattering of the type that arises because of the recoil of photons that collide with molecules in a homogeneous medium, is known as Rayleigh scattering. yThe energy of the scattered photons, in this case, is very nearly equal to the energy of the colliding or incident photons. For this reason, very little change in wavelength occurs in YRayleigh scattering. Scattering that arises because of the presence of motes in a lliquid or gas is known as Tyndall scattering. No substantial change in wavelength occurs in Tyndall scattering. Similarly, scratches and other small irregularities in or on the cell walls also produce scattering without change in wavelength. i

The Raman scattering process is of an entirely differ'- ent kind from either the Rayleigh or the Tyndall A'scattering process. In the Raman process, photons of exciting radiation are absorbed b'y molecules or otherwise inter'- act'with them in 'such a'way that a quantized exchange of energy occurs. lIn any event, after the interaction has occurred the energy of the photon is altered, either being increased or decreased by amounts that correspond -to certain differences .in the energy levels characteristic of the molecule. Assume, for example, that the frequency of the exciting -radiatiorris f1 and that a frequency corresponding to a specific difference of energy levels of the lolecule involved in the interaction is fo, then if such Due to the fact ECC interaction occurs, the photon that is emitted from the molecule has a different energy corresponding to a different frequency f2. Thus, the interaction may result in the change of energy of the indicentphoton hfl to an energy hjgl of the emitted photon expressed by the following equation: l

The plus sign is employed when the incident photon absorbs energy Vfrom the molecule and the minus sign is employed when the incident photon loses energy to the molecule. From this expression, it is apparent that a spectrum that is characteristic of the molecule can be ascertained in terms of the differences in the frequencies (f2) of various Raman lines and the frequency (f1) of the exciting radiation. In view of the fact that the energy levels of a molecule are quantized,v these frequency differences Ahave a series of discrete values which characterize the different Raman lines. lit is to be noted that the Raman lines do not have any fixed frequencies, but that their frequencies depend upon the frequency of the exciting radiation. However, the difference between the frequency of each Raman line and the frequency of exciting radiation is independent of the frequency of the exciting radiation. y v n Generally speaking, the frequency differences that are thus' characteristic of Raman lines are the same as the frequencies of lines that appear in the infra-red absorption spectrum or emission spectrum of the same molecules. However, the frequencies characteristic of the absorption or emission spectra are in effect added to or subtracted from the frequency of the exciting radiation. Further more, the frequency ofthe exciting radiation employed in Raman spectroscopy -is high compared with the frequencies present in the infra-red spectra of the molecules subjected to such excitation. For these reasons, the spread of frequencies of a Raman spectrum expressed on an octave basis is less than the spread of frequencies of an absorption or emission spectrum. This fact alone makes it Vadyanta'geous to employ Raman spectroscopy rather than infra-red spectroscopy in the analysis of molecular compositions, since by the Vuse of Raman spectra produced when the exciting radiation has a short wavelength 'such as the wavelength of blue, or ultra-violet light, an entire Raman spectrum occurs over a range less than one octave, whereas lthe corresponding infra-red absorption or emission spectrum occurs over a range that greatly exceeds one octave. By virtue of the reduced frequency range produced it becomes a simple matter to observe the various lines of a Raman spectrum with a single dispersing element, Whether it be a prism or a grating.

Not all infra-'red lines have their counterpart in Raman 'spectra and not all Raman `lines have their counterpart n infra-red spectra.l Generally speaking, though, Raman spectra are simpler than infra-red spectra of the sarne molecules.

Another advantage of Raman spectroscopy resides in the fact that by employing excitation radiation of the proper frequency, the Raman vspectrum lines are caused to fall in the visible or near ultra-violet spectral regions for which there exist exceptionally eicient'photodetector's and 'optical elements.

Fluorescence spectra also appeared in scattering processes. ,In .uoresce'nce, an incident photon is absorbed by a molecule. At a later time, the excited molecule which has vpreviously absorbed such a photon falls to a lower energy state, thereby emitting a photon.v If a molecule returns to the same state that it was in prior to the absorption of the photon, the iluorescent radiation emitted willhave thel same frequency as the exciting radiation. However, if, as is more usual, the molecuie by fluoroescence or Tyndall scattering. order to achieve that goal, a tubular sample chamber 'employed for holding a sample.

enters a diterent energy state, then the fluorescent radiation emitted has a different frequency. In order for fluorescence to occur, the exciting photon itself must have an energy corresponding to a permitted change of energy characteristic of the molecule. In contrast, the Raman effect occurs irrespective of the frequency of the radiation.

Another advantage of employing Raman spectroscopy 'compared with infra-red spectroscopy lies in the fa'ct that the intensity of the Raman spectrum of each molecular species of a mixture is ordinarily proportional to the concentration of that species in a mixture. This fact simplies the quantitative analysis of mixtures. This simplication does not ordinarily exist in infra-red absorption spectroscopy. For the foregoing reasons and others, it is apparent that Raman spectroscopy is really an important instrutant in industrial applications where it is desirable to invention, a Raman spectrophotometer of increased sensitivity is provided.

In the past, one of the limitations on the sensitivity attainable in Raman spectroscopy arose from the fact that care was always exercised to avoid transmitting to the monochromator any radiation that is emitted from or scattered by the walls of the sample chamber such as In the past, in

having an opaque curved horn at the open end was (See, for example, 4Practical Spectroscopy by Harrison, Lord, and Loofburow, Vpage 509, Prentice Hall, 1948). Exciting radiation entered the sample laterally through the part of the Y tubular member that was not so coated. In addition,

the optical system was so designed that two aperture stop images were formed in the exposed part of the sample,

one aperture stop image being located near the closed end of the sample, and the other aperture stop image being located adjacent the edge of the opaque coating, and such aperture stop images were formed in the cross-sectional .area of the sample within the-sample tube. In this way, the only radiation transmitted to the monochromator was that which originated in the liquid sample and all `o f the radiation originating in the body of the cell wall or at its surfaceswas excluded from the monochromator. In this connection, it is to be noted that the curved end or horn of the tube was curved sufficiently far to trap exciting radiation that was transmitted or scattered in its direction and thus preventing it from entering the spectrometer.

According to the present invention, the sample is illuminated with exciting radiation over an extended region .thereoffar beyond that illuminated in prior art systems. In particular, the sample is illuminated over a region sufficiently long to enable many reflections to occur along the path of travel of radiation which is transmitted to the monochromator. Unlike former systems, in the Raman spectrophotometer of my invention, the monochromator thus sees the walls of the sample chamber. By virtue of this fact, the intensity of the Raman spectrum `transmitted to the monochromator is increased since the intensity of the Raman spectrum is proportional to the length of the sample tube illuminated by the exciting radiation. At the same time, a background of undesired radiation emitted from the walls of the sample cell is also transmitted to the monochromator.

I have discovered that the effects of this background 'radiation can be greatly reduced and often substantially eliminated by employing a double monochromator to disperse the radiation emerging from the sample cell. Thus, in my'invention, a double monochromator is employed -mentfor research, whether it-be' pure research or infdustrial research. Raman spectroscopy is also importo reduce the intensity of undesired or background radiation compared with the intensity of the spectral lines being measured. Such a monochromator reduces the intensity of the undesired'radiation in several ways. First of all, it provides a monochromator of high resolving power to facilitate the separation of the lines of the Raman spectrum from the line representing the exciting radiation. But more important, the use of the second section in the double monochromator greatly reduces the intensity of lany background radiation that enters the entrance slit and is transmitted to the exit slit of the first section of the monochromator because of scattering inthe rst section. f f

In practice, I have found that the background radiation that enters themonochromator from the sample cell is due primarily to the scattering of exciting radiation from the sample and from the walls of the sample chamber. In effect, the background radiationconsists of a feeblev background of white light and a strong component of the exciting radiation. In accordance ywith this invention, the scattered exciting radiation that reaches the exit slit as the spectrum i's scanned is greatly attenuated by the use of a'double monochromator, leaving a small residual background spectrum upon which the Raman spectrum from the sample is superposed. For this reason, my invention makes advantageous use of a sample which extends over a region through which the monochromator sees the walls of the sample chamber. In order further to increase the effectiveness of my invention, a double monochromator of the type that `employs gratings as dispersing elementsis used. Such a double-grating monochromator may be readily rendered substantially free of ghost transmission from the entrance slit to the exit slit. Even though some residual scattered radiation is transmitted from the entrance slit to the exit slit of the double monochromator by multiplyldeflected paths, such residual scattered radiation is very feeble, thus making it possible to detect Raman radiation from small samples.

By making it possible to utilize radiation from a longer Vsample length and from a wider portion of a cylindrical sample tube in accordance with this invention, the sensitivity of the Raman spectrograph embodying the invention is greatly increased. For this reason, this invention may be employed to determine the Raman spectra of much smaller samples than could be effectively employed heretofore.

To achieve such increase in sensitivity, radiation emerging from the end-of a tubular sample cell of a Raman spectrophotometer is `utilized over a larger portion of the cross-sectionall area of the cell and radiation is also utilized from a longer length of the cell. In addition, some other features are employed to accomplish this improvement. In order to understand how a larger portion of the cross-sectional area of the sample is utilized, it is necessary to recall that in any monochromator the limits of the useful beam are defined by two aperture stops. One of these aperture stops is normally formed by the entrance `slit and the other is normally defined by the limits of the dispersing element, or the edges of a mask adjacent thereto. The heights and Widths of these two aperture stops and the distance between them define in effect a vertical and a horizontal Lagrange constant. In the past, in Raman spectroscopy, an image of the entrance slit which was usually very narrow and an image of the other aperture stop, which was usually about square, were formed within the sample cell, thus forming a truncated pyramidal sample region that was square at one end and was rectangular at the other. In such a system, the only Raman radiation that Was utilized in the monochromator was that which emerged from the parts of the sample that were located within the pyramidal sample region. Radiation emerging along rays passing alongside the narrow .to horizontal and .drawings are' provided only to' illustrate the invention and that the invention is not limited -to such details, but that the scope of the invention is defined by the appended claims.

Figure 1 is a schematic diagram of a Raman spectro- :photometer employing a single-slit monochrom-ator and vembodying this invention; Fig.Y 2 represents a Raman spectrogram of the type Aobtained with the Raman spectrophotometer of this in- ,vention f Fig.`3 is-a vertical sectional view of the source unit of the Raman spectrophotometer of Fig. 1; f Fig.' 4 isa plan view of an optical layout showing the reiative positions of the 'various parts of the entrance system of the spectrophotometer;

Figs. 4a and 4b are isometric views of parts of the entrance system;

Fig. 5 is a vertical cross-sectional view of the horizontal image slicer as viewed along .the optic axis;

Fig. 6 is a vertical cross-sectional -view of the vertical image slicer as viewed'looking toward the sample along the opticaxis;

Fig. 7a is a side elevational view of one form of the first image slicer;

Fig. 7b is a side elevational view of another form of the first image slicer;

Fig. 8 is a schematic diagram of a Raman spectro- -photometer employing a. twin-slit `monochromator and -embodying this invention;

Fig. 9 is a vertical sectional view of the first image slicer employed withv theV twin-slit monochromator;

Fig.10 is a vertical-sectional end view of the second image slicer employed -with the twin-slit monochromator; Fig. ll 'is a cross-sectional view of a sample cell;

, Fig. 12 is an isometric diagram employed in explaining the cell optics;

Fig. 13 is a cross-sectional view of an alternative embodiment of the sample cell; and

Fig. 14 is a schematic diagram of the single-slit monochromator employed in explaining the action of the field tiattener.

In` the Raman spectrophotometer illustrated in Figure l andv -embodying the invention, scattered radiation emerging from a source unit along the optic axis X-X is transmitted through a Lagrange-constant transformer uni-t -through an entrance slit SO into a double grating monochromator Monochromatic radiation emerging from the exit slit SE of the monochromator 40 strikes a photocell'70. Currents generated by the photocell 70in proportion to the intensity of radi-ation transmittcd thereto by the monochromator, are amplified by means of an amplifier 80 and then applied `to an automatic recorder 90 inwhich a spectrogram of the Raman vspectrum emitted from `the sample in the source unit recorded, lnithis spectrophotometer, the image converter unit 30.that includes a Lagrange-constant transformer is employed to convert a beam having substantially square cross-section throughout the sample to a beam which is tall and narrow in part and which may vltherefore conform to the shape of the entrance slit So. yBy using such an image converter unit, it lbecomes possible to employ very small Raman cells which are circular or square in cross-section in the source unit, and thus to measure or record a Raman spectrum of a sam- Iple even though only a very small quantity of that sample is available for analysis.

The system is characterized by two principal planes. Cnc pricipal plane, which includes the optic axis, is vertical. The other principal plane is horizontal. The heights of the slits SO, SI and SE are parallel to the vertical principal plane, while the widths are parallel to the other. While the invention is described with reference to only one orientation of the principal planes relative verticalplanes, other orientations are :possible Y v 8 1HE MoNoCHRoMAToR The two monochromator sections 42 and 44 are of similar construction and are on opposite sides of a central walll 46. An intermediate slit SI located in wall 46 serves-as the exit slit for the monochromator section 42 and as the entrance slit of the second monochromator section 44. Heterochromatic radiation that is transmitted from the source unit 10 through the entrance slit S0 impinges upon Ia collimating mirror 48 which deflects the radiation as a collimated beam to a first grating 5. Heterochromatic radiation diifracted by the grating 50 returns to the collimating mirror 48 which reflects the radiation to a mirror 52 which in turn reflects the radiationto the plane of the intermediate slit SI where radiation of different wavelengths is brought to focus at dif- -ferent points by the collimating mirror 48. Monochromatic radiation selected by and passing through the n'- termediate slit SI is reflected by a mirror 54 toward a collimating mirror 56 which reects the radiation as a collim-ated beam toward a second diffraction grating 58. Monochromatic radiation of the selected wavelength and dilracted 4by the grating 58 is returned to the collimating mirror 56 which focuses this monochromatic radiation at the exit slit SE.

In the double monochromator illustrated, each monoychromator section is of the Littrow type in which a single collimating element is employed to transmit radiation toward the corresponding grating and to receive radiation diffracted therefrom. In this double monochro- -mator, the two gratings are inclined asymmetrically with respect to the beam even though the paths are symmetrical. With this arrangement, the dispersions produced by the two gratings add, thereby increasing the overall dispersion of the monochromator and aiding in achieving high resolving power.

The two gratings 50 and 58 may be in the form of identical replica gratings. Their faces are vertical and -they are so mounted relative to the vertical principal plane that both monochromator sections 42 and 44 are capable of transmitting monochromatic radiation of the same wavelength therethrough. Both gratings are mountved on the same rotatable frame and at such angles that the normals lto both gratings are always inclined at the same angles with respect to the collimated beams incident thereon.

Each of the gratings has a useful aperture that is about 4square and is about 10 cm. on each side and each grating is ruled with 12,000 lines/cm. and is blazed for 4500 A. in the first order. The two gratings are rotated in unison by means of 'a control unit 60 so as to vary the wavelength of the monochromatic radiation emerging 'from'the exit slit SE. The control unit 60 is also employed to drive a strip of recording paper in the recorder while the lRaman spectrum is being scanned so as t0 produce a Raman spectrogram of the type in which ab- 'scissae represent frequency and ordinates represent beam intensity. A section of such a spectrogram is illustrated in Fig. 2. In this spectrogram, the intensity (I) of radiation received by the photocell as the spectrum is scanned is plotted as a function of the frequency differential )c1-f2 where f1 is the frequency of the exciting radiation and f2 is the frequency of the Raman radiation.

THE SOURCE UNIT As indicated in Fig. 3 the source unit 10 comprises a yhelical gaseous discharge tube 12 mounted around and .co-axially witha capillary sample tube 14 that is of cirif pumped through the housing by means or a fan 21 manga an inlet 22 exits from the beine unit zu. ir serves to cool the helical tubeV 12, the lter vessels I6, and the sample tube 14. The baille plates 24 are aranged transverse to the opening 23 at the outer end of the baffle unit 20 so as to permit free flow of cooling air therethrough without however permitting external radiation to' be transmitted inwardly of the opening 23 except in greatly attenuated form b'y devious multiple paths past the black baiiie plates. The baille unit 26 and the surfaces or the baffle plates 24 are coated with light- Vabsorbing material such as black paint so as to minimize the amount of radiation transmitted bythe baille unit 20. Hwci/er, the interior surface of the housing 18 is coated with a highly reflective material such as aluminum paint to increase the amount o'fexcit'ing radiatin passing into .the sample cell. The baille unit 20 is made removable s as '-t' facilitate installation and removal of sar'nple c ells and is normally held in place by means of a screw I9.

The sample cell 14 is in the form of a Astraight capillary tube that is normally closed at one end and open at the other. The closed end is' supported in a conical aperture 25 of a tubular support member 26 removably mounted on the inner wall 2S of the housing 18. The open end 31 o f the sampletube rests on a supprt arm 32. The sample tube is held in placezand its closedend is pressed into the conical aperture 25 by means of a spring 33l acting on a circular disc 34 mounted at the inner end ofa rod 36 that is slidably arranged in a passage 38 of the support arrn 32. The vrod 38 is profvided with a knurled knob 40 at its outer end to facili# tatel installation and removal of sample cells.

The' tube 12 is in the form of a low pressure mercury lamp that is energized by means of electrical power supplied from an external source. As is well known, such amercury lamp emits a line spectrum consisting of many monochromatic lines. The filter vessels 16 contain iiuids of a suitable type to transmit to the sample cell 14 substantially monochromatic radiation emitted by the mercury vapor lamp. In the ltering action sonic absorption due to the walls of the vessels 16 also occurs and aids in the' filter action. In a specific embodiment of .the invention which has been employed use has been made of the Hg lines near 4358 A. Hg as exciting radiation. l

-In order to reduce confusion and annoyance that wuld arise if two Raman spectra caused by two or more monchromatic exciting lines were superposed, the filter 16 is made up of two sections, one containing a sodium nitrite solution and the other containing a cupic arn- `rnoniurnv sulfate solution. Such a filter connues the 4exciting radiation transmitted from the mercury lanip 12 to thefsample cell 14 to the mercury lines having the wave# length near 4358 A., where the chief component has the wavelength of 4358.35 A.

When such monochromatic exciting radiation strikes molecules of a sample contained in the sample cell 14, it is scattered in all directions. Due to the Raman effect, parts of -the scattered radiation have a different wavelength from that of the exciting radiation. Such scattered radiation, as explained above, is quantized in accordance with the energy levelsno'f the scattering mole'- cul'es. However, as a result of Tyndall or other scattering sonic of the scattered `radiation is of the saine er nearly the same wavelength as the exciting radiation. The intensity of the Raman `radiation is veryrsfmall compared to the intensity of other scattered radiation. In this invention, scattered radiation of both types is transmitted to the monochromator and a Lagrangeconstant transformer is employed to increase -the efficiency with which the Raman radiation is transmitted into the entrance slit 0 and v.through the monochrrnator 40 `to the -photo'cell 70. The manner in which the scattered' Y 4le radiation is transmitted frorn` the closed end of the Sample lI I4 f6 ff- Slif S0 f Ill-flff iS explained in detail below.

LAGRNGELCONSTANT TRANSFRNIER UNIT In the Raineri spectrophotometer of Fig. 1, scattered radiation which transmitted through square aperture stop images Ag and S3 within the tube 14 is focused as an elongated image at the entrance slit S0 of the mondchromator. In order to trace the paths along which the rays travel from the' s'arnple tube through the m'nochromator, it is sometimes convenient to trace the paths in the reverse direction. Such a reverse type of analysis is permissible because of the rcversibility or reciprocity relationship 'which applies to the geometrical optics prop# ert'ies of the system.

To trace the rays in the reverse direction, assume that radiation enters the exit slit S and travels along certain paths adjacent to the optic axis X-X, striking the following elements' in the order mentioned: collimating mirror 56, the reflecting mirror S4, the intermediate slit SI, the reflecting mirror 52, the collimating miuror 48, the diffraction grating 50, the collimating mirror 48, and their the entrance' slit So and that the rays then travel through the Lagrange transformer unit 30 to the sample cell 14. In the passage of rays through the monochrdmator the diffraction grating 50 carries an interior aperture stop which defines the lateral limits of thebern at the grating. In the particular system described herein, it is Iassumed thatthe aperture stop of the monochromator is defined by the face of the diffraction grating 50 or by means of a mask mounted on it. In the following description, however, the aperture stop referred to is the apparent or virtual aperture stop defined by they virtual irnge A f the diffraction grating 50 that is formed by the collimating mirror I48 on the opposite side thereof from the entrance slit SO. For convenience in explain ing the invention, it is assumed that the apparent aperture' stop is nearly square having an aspect ratio near unity, even though in practice the aperture stop departs somewhat from a square shape and even changes in size and shape somewhat as the wavelength of the radiation transmitted through the monochromator is altered.

The relative positions of the virtual image A() and the entrance slit So together with the relative positions of parts of the Lagrange-constant transformer unit and the sample syste'rn are indicated in Fig. 4 and Figs. 4a tibt The relative positions of the various parts of the 'omti cal system and theirniost important optical characteristics are summarized in lTable I, where each element or image Iis identifiedi'n' the first column, its distance D from the entrance slit SO is indicated in the second column, its horizontal dimension or characteristic h in the horizontal principal plane is indicated in the fourth column and its vertical dimension or chanacteristic v in the vertical principal plane isindicated in the fifth column. Horizontal dimensions of images, slits, and the like are thus listed under the fourth column, while vertical dimensions of images and apertures are indicated in the fifth column. Focal lengths of elements in a horizontal plane are also thus listed in the fourth column, while vfocal lengths the Vertical plane Iare tabulated in the fifth column. In the sixth column, the thickness t of the respective lenses along the optic axis X-X is indicated.

The various yelements or images are listed in Table I consecutively in the order that they lie along the optic axis X-X. Distances D between successive elements or images are listed in the third column. Where the position of a lens is indicated, it is to be understood that the measurement is to be made from the Gaussian or unit plane of the lens. Distances along the optic axisX- X and focal lengths stated in inches ("),while dimensions, such as image widths and .the like, transversel to the optic axis are stated in millimeters (mm.).

Table I I lElement or D AD h v t Image Ao -100 250 mm. 250 mm. 99.6l

`Lol .36 o 3 H 2l 21 0.376

6 Sn 0.0" 1.0 mm. 100 mm.

5.12" Lx 5.12" 3.76" m 0.125

' 3.20 An, 8.32 1 1 H 10.2 mm.

1 4 S11,A 19.46 2.85 111m.

Lt 19.46" 4.51" m 6.125"

1.187 A 27.35" 6.42 mm. 64.2 mm.

0.125 G1, G2, 27.60" 8.63 0.126 r 12.50" Y L4 39.98 12.63" en 0.187"

0.19 8 40.16 '0 50" 5.10 mm. 51.5 mm. H., H., 40.66" 12.63" 0.56" 4 i 0.24"

' 17.1Il La 58.0 0.75 0.75 0.250" 0.61"

A: 58.6" 2.0 mm. 2.0 mm. 0.157

Sx 58.8" 2.0 mm. 2.0 mm.

` Referring again to Figs. 4, 4a and 4b, and .to Table I, xt is to be noted that the aperture stop AO is located .100" to the left of the entrance slit S0. The height 4of the aperture stop in a vertical plane is AOV-:250 mm.

the width of the aperture stop in a horizontal Aplane is The aperture stop A0 thus has an aspect ratio of unity.

The entrance slit SO is located at the origin of the .abscissae which defines the positions of the other elements. The height of the entrance slit is Sov=l00 mm.

and the width of the entrance slit S0 is S0h=1.0 mm.

The-aspect ratio of the entrance slit is thus 100.

Two maximum Lagrange constants exist in the region constant l...r is expressed by the following equation:

Lv d

and the maximum horizontal Lagrangei'constant Lh is expressed by the following equation:

' d==distance from virtual image of aperture stop Aoto the entrance slit So.

In this particular case, the values of these Lagrange constants are approximately Lv=1.0 cm. Lh=0.01 cm.

lt is to be noted that the maximum Lagrange constants in the two principal planes bear a ratio of 100 to` 1,

vthat is- 'vertical magnitications.

12 Ordinarily, a high ratio of maximum. vertical vto horizontal Lagrange constants occurs in a monochromator 4because of the fact that the entrance slit is tall and narrow and the interior aperture stop, whether it be defined by a diffraction grating or by a prism or by the collimating mirror or by some other element of the monochroma- Itor is generally about as wide as it is high. Prior to this invention, unequal maximum vertical and horizontal Lagrange constants also existed in the sample region,'but as will be seen hereinafter the Lagrange constant transformer 30 causes radiation transmitted along paths that fill a pair of square stop images in the sample region .to pass through the monochromator along .paths that ll the stops A0 and S0. By rendering the maximum vertical and horizontal Lagrange constants equal in the 'sample region more ctlicient use can be made of the radiation scattered from that region thereby making it pos- -"sible to analyze smaller samples effectively than would be possible otherwise.

A field-flattening lens L0 is located adjacent the entrance slit on the internal side thereof. This lens, which is spherical, has a focal length of FOh=F0,=2`1.0"

The lenses L0 and L1 focus a horizontal image Au, of the aperture stop Ao at a distance 8.32" from the entrance slit S0 and a horizontal image Slh of the entrance slit S0 at a distance 19.46 from the slit S0. The width 4of the horizontal image A11l of the aperture stop so formed is Anl: mm.

Thewidth of the horizontal image of the entrance slit S0 so formed is S1h=2.85 mm.

A second cylindrical lens L2 having a vertical axis is located at the position of the horizontal image Su, of the entrance slit. This lens, which has a thickness of 0.125" has a horizontal focal length A spherical lens L3 is located 26.16" from the exit slit S0. This lens has a thickness of 0.437" and a focal length The lenses L2 and L3 form a horizontal image Agg of .the aperture stop A0 at a position 27.35" from the slit lThe height of the image so formed is vIt is important to note that the lenses L0, L1, L2 and L3 thus act as a rst anamorphoser having an anamorphotic ratio of 10. Such an anamorphoser forms an image A of the aperture stop AOl at the position of the component images A21, and Alv, but with different horizontal and The vertical magnification is 0.257 while the horizontal magnication is 0.0257. Even though the ratio of the height to the width of the image A is only l0 to 1, nevertheless the vertical and horizontal Lagrange constants applicable to radiation entering and emerging from that image have the same values as in the monochromator.

A rst image slicer R1 comprising a first series of andasse G10, is located at the' position of the image A. Each of these lens segments designated generallyby theY lletter G is in the -fom of a cylindrical lens having a vertical axis. The focal length of each' of the lens segments is 'Each of the lens segments has a height which is 1710 of the height A1,v of the image A of the aperture stop,

that is, the height of each of the lens segments is 6.42

The various lens segments G1 .Y G10 are ar'- ranged in a staggered array such as an echel'on array in which the centers of curvature of the lenses in directions parallel to the' optic axis are spaced apart horizontally by 3.41 inni.

The cylindrical lens segments G1 G11, slice the image into ten equal parts A1' A111 -each of which is square, having a height and width of 6.4 inni.,

mm. apart, but that they all lie in the same horizontal plne- In practice the spherical lens L3 and the set oflens segments Gtarecementedto theopposite faces of a block or plateofwglass B1, which is shown in Figs'. 4 and 70 but is omitted fromFig, 4a for the sake of clarity. The block of glass is 0.750 thick and the distances given in Table I are those employed when the block of glassI is present.. If it is omitted, all distances from Sgreater than 26.16'7' should be reduced by 0.26". 4 This block of may, of course, actually be inthe formi kof a reflectingprism if forl any reason itis desiredto introduce a change in the direction of the optic axis X-J-X at a point between the positions of the lens L3 and the lens segments In the latter case,kthe opposite faces are the twoufaces of the prism which intersect the rays along which light is transmitted. To form such a composite lens, the spherical lens L3 is of the plano-convex typeaand eachof the segments G is also of the plano-convex type. n Such a cornposite image slicer in which the block B1 is in the form of a rectangular-parallelopiped isillustrated in Fig. ,7a While such an image slicer employing a 45 isoceles prism yis shown in lFig.` 7b; If desired, the segments G may be cemented directly to the lens L3 if no rotation of the optic axis is desired. In any event the` lens segments are cemented to each other and to a glass element to form a rigid unitary image slicer. e I y e y Y H 4.A cylindrical lens L4 having a vertical axisais located 40.0 from the entrance slitl S0.Y This lens has a thickness of 0.187" and a horizontal focal length Each of the composite lenses formed Iby the element L3 and one of the respective segments G, together with the lens L4, forms a horizontal image S21l of the horizontal image S11, that is located at the Ilens L2. spherical lenses falso form a vertical image S1, of the entrance slit SO. The two images so formed by each of the segmental spherical lenses are brought to a focus at the same position 40.16" from the entrance slit S combining to form an image S of the entrance slit S0. Each of the images S so Aformed has a height S1v=52.0 mm.

and a Width l S2h=`5.1 mm.

Each of these rire tenses' i1, L2, L, L1, and acti of segiiieiiis' o, rhin acts" aslv a seo'iid -namor'phosr Having ajnj aiiiiioi-y photic ratiov of 0.1 Where' z' has each of the values I, .2, 3 10i. Such ari anamorphoser fomis'an image S1 of the entrance slit S` at ythe pesitionrof the ,component 'images (S1, andrS211, butwith different horizontal and yeti'- ticl magnification'. vThe `vertical magnification is 0.52, while the' horizontal magnification is 5.1. a The anamorphoser f orinedby eachof the lens segments G1 .A vrG10 transmits light that passes through the .COriesponding square image parts A1 A111' to" av diier'fentwpositin. Thus, the various images S1, S2f' S111 of the eli'- trlarlce slit SO formed by the different anamoiphoslers including the corresponding lens segments G1 AG11, are deiiectedhorizontally transverse to theoptic Xz-X because of the fact that the centers of the lenses G1 L G10 are displaced horizontally. By way, of example", Vthe radiation that is transrnitted from the slit S5 through ythe square -image'party A1" forms an image S1', Vwhilevthe radiation that isgtransmitted through the square image vpart A1 'forms the image S2, etc. The horizontal displacement of the centers of the lens segments G are so chosen that all ten images S1', S1 S10 lie side by side with successive images contiguous, thereby forming a composite square image Sc having ia height of the. individual images S'. 1 I f A second image slicer R2 is `located approximately at thevposition' of the Vcomposite image Sc' so formed of the entrance slit SO'. This image slicer comprises ten cylin drical lenssegments H1, H2 H10'. 4 1 l v .a The various segments H of the second image slicei'` and the lens L4 may `be cemented to opposite facesofal glass blockin a manner similar to that` by which the lens L 'g andthe segments G1 are cemented to a glass .bloclvis shown in yFig. 7b In this case too the `glass block may be omitted.- v Y Y, l y

The, ten cylindrical lens segments H1 H10' of ,the second image 'slicer R2 have widths equal to the widths of thevindividual image S', the widthjof each of the lens segments in a horizontal direction being S2h=5.1 mm.

flfherectangular image S1 that is formed by the cylindrical lens segment G1Yis formedv at the face of corresponding cylindrical lens segment H1 where i again has each of the values 1, 2, 3 10. Thus, for example, the fectangular image S1 that is formed by the cylindrical lens segment G1 is formed at the lface of corresponding cylindrical lens segmentV H1. The rectangular image S2 that is formed by the cylindrical lens segment G2 is formed at theface of corresponding cylindrical lens segment H2 and so on.

, Two spherical lenses L5 `and L- are arranged between the secondtixnage slicer R11 and the sample cell 14., The iirst lens L5 is located 40.9, yfrom the entrance slit So. This lens has vertical and horizontal focal lengths which are equal, namely, the focal length The second lens Le is located 58.0 from the entrance slit So. This lens also has vertical and horizontal focal length which are equal, namely, the focal length It will be noted that the horizontal focal length of the cylindrical lens L4 is equal to the vertical focal length of each of the lens segments H1 H10, so that in eifect each of the lens segments H1 H10 combined with the cylindrical lens L1 acts as a spherical lens. The lens segments H1 H111 are so located that each of the spherical lenses which it forms with the lens L1 forms an image of the corresponding image part A1 n. A111 at the same position within the sample cell 14. kAllff the imagesso formed inthe sample cell are square since they havefthe same shape as each of the square seg"- inents A1' A10' and allof these square images ae require that superposed and coextensive vforming a single square im- `age A3 of the square verticalaperture stop Ao. To produce such superposition the centers of successive lens segments H1 H13 are spaced apart vertically by -1.94 mm. Thus, for example, more particularly, the spherical lens formed by the cylindrical lens'segment H1 and the cylindrical lens L4 and the spherical lens L5 and :L3 form a square image A3 of the image part A1 in the sample cell 14. Likewise, each of the other vertical cylin- ,drical lens segmentsH3 H10 forms an image A3 of Athe corresponding image section A3' A13 at the same location in the sample cell. -The two lenses L5 and L3 form an image S3 of the composite square image SO. The image S3 is also formed in the sample cell 14, and it too is square. Each of the "images A3 and S3 has a height of 2 mm. and a width of '.2 mm.

'In Table I the positions of the aperture stop images A3 fand S3jhave been indicated on the assumption that no `4sample cell is present. It can be shown that if a sample 'lis present, then the sizes of the aperture stop images A3 and S3 are unaffected, but their positions are changed, "more particularly the distance between images A3 and S3 vis proportional to the refractive index n of the sample. In practice, therefore, the face of the sample cell 14 is located adjacent the front aperture stop image A3 so that all of the radiation that is transmitted to the monochro- 4frnato'r travels along rays that emerge from the end face lof-*the cell, and not ialong rays which intersect the outer sidewall ofthe cell.

In the sample cell, both the maximum vertical andv horizontal Lagrangeconstants have equal values, since `the product of the widths of the two images A3 and S3 Ajequals the product of the heights of these images. By lmeans of the image slicer illustrated herein, the maximum been reduced by a factor of in the transmission of light I"through the two image slicers R1 and R3, and the maxi- ',mum horizontal Lagrange constant of the monochromator has been multiplied by 10, thus rendering the two maximum Lagrange constants equal in the sample cell. In effect, then, the optical system provided herein conthe system.Y Y

Expressed mathematically, the laws of thermodynamics nLvLhL-constant thro'ughout'the entire beam from one end of the instru- Ament to the other where lthe terms L,l and L11 are the maximumyertical and horizontal Lagrange constants in air as previously defined herein.Y This equation assumes lthat theA cosine of the angle of the beam is nearly equal to unity. #If this condition is not:V met, the cosine terms must be introduced.

It canr'be shown that if the ratio of the maximum verti- -call to the maximum horizontal Lagrange constants inthe monochromator is N2, that is if then. to` produce equality of maximum horizontal and maximum vertical Lagrange constants and square aperture stop images in the sample cell, the number of lens segments employed in each of the image slicers S1 and A,S3 should be N/M..v `It. can be shown that the ratioof `vertical Lagrange constant of the monochromator has Generally speaking the lenses which form the image A' of the aperture stop at the first slicer R3 anamorphoses the aperture stop by a anamorphotic -ratio N vertical magnification of aperture stop A0 horizontal magnication of aperture stop A0 andthe optical elements which form each of the images `S' of the entrance slit S0 at the second image slicer R3 'anamorphoses the image of the entrance slit S0 by the anamorphotic ratio Y 2l/:: A vertical magnification of entrance slit N horizontal magnification of entrance slit In order to utilize as small a sample as possible effectively and economically, a sample cell of circular or jsquare cross-section is employed. Greatest use'of radiav'tion emerging from such a sample cell is obtained by utilizing square stops A3 and S3 for which M :1. In the particular case specifically described above N=10. Accordingly, ten segments were employed in both image Slicers.

By employing a number of lens segments in eachof the l,two slicers other than N, Lagrange constant transforma- "tion may still be achieved with the single-slit monochromator described above, but in this case the maximum `Vertical and horizontal Lag-range constants at the sample cell will not be equal. It will also be understood that if fthe number of lens segments in the two Slicers are unequal, some advantages of this invention may be achieved 'even though one or both of the images in the sample v'region is not square. In the foregoing detailed description of the optical system, the paths along which radiation would travel through ,the monochromator and then to the right through the Lagrange-constant transformer and then into the sample 'cell 14 have generally been considered. If light were to travel in this direction, then all light traveling through the space within the boundaries of the aperture stops A0 fand through the entrance slit SO would also travel through r two square aperture stop images A3 and S3 in the sample cell. jDue to the principle of reversibility, it is clear that Aall radiation which is scattered by the sample cell 14, and `which passes to the left along paths that extend through the square image area S3 and through the square image `area A3, is transmitted along the same paths as pre- "viously described through the Lagrange-constant trans- YformerI and through the entrance slit SO and throughr'the aperture stop A0. This is sometimes referred to as direct travel.

Considering reverse travel of radiation, it will be noted S3', S4', S5', S3', S7', S3', S9', and S111 of the image Sc are all focused as coextensive images over the area of the entrance slit S0. ln other words, considering reverse travel, equal horizontal slices of the aperture stop AO are focused in coincident position at image A3, and considering direct travel, equal vertical slices of the aperture stop L17 image S3 are focused at the entrance slit S0. Thus, different horizontal slices of the aperture image stop AO are conjugated with respect to a square image A3 in the sample region and dierent vertical slices of a square composite slit image S3 in the sample region are conjugated at coincident positions at the rectangular aperture stop S0.

Looked at in another light, the two lenses L and L3 image the image area S3 at the image slicer R3 as a square image S3. Furthermore, each of the vertical lenses H1' forms a square image A1 of the image area A3 at the corresponding horizontal lens G1 of the rst image slicer R1, where i=l, 2 l0. Thus, for example, the vertical lens H1 forms a square image'A1 of the image area A3 at the horizontal lines G1. Similarly, the vertical vlens H3 forms a square image A3' of the image area A3 at the second horizontal leus G3 and so on.

In this way, astack of ten square images1A1', A3' A13' of the image area A3 are arranged vertically one above the other at the first image slicer R1, thus forming a composite rectangular image A having a height ten times its width. The anamorphoser comprising the lenses L3, L1, L2, and L3 forms a square image of the rectangular image A at the aperture stop A0. At the same time, the anamorphoser comprising lenses L1, L3 and L4 and each of the horizontal lenses G1 forms an image of the corresponding sections S1 of the image Sc of the area S3 at the entrance slit S0, where i=l, 2 l0. Thus, the image segment S1 is focused by means of the lenses L1, L3 and L4 and by the horizontal segmental lens G1 at the entrance slit S0. Likewise, the image segment S3 is focused by means of the lenses L1, L3 and L1 and by the horizontal segmental lens G3 at the entrance slit SO, and so on. Each of the ten images so formed are coextensive with the entrance slit SO. It is interesting to note that this arrangement is entirely different from that employed by Bowen and from that employed by Benesch and Strong, since in each of their devices only one slicing and stacking operation occurs. But in this invention two aperture stops or their images are sliced and stacked, ,and the two slicing and stacking operations occur in transverse directions.

in the present illustrated system, two sets .of aperture stop images are formed. One of these sets includes the image A3 and the image A of the virtual aperture stop Ao. The other set includes the image S3. and the image Se of the aperture stop .established 'bythe entrance slit So. The images A and Sc are located at the first and second slicers R and Rv respectively. in this system, the aperturestop image that is nearer the end .ofthe sample tube ldfrom which the radiation emerges is square and is real. The other aperture stop image S3 is also sqzuare and real. ln the specific form of the invention described above, the two aperture stop images are of equalA size. However, the aperture sto'p limage S3 may be foimedat other points on either side of the rst mentioned aperture stop A3 and may be either a real image or a virtual image. lt can be shown that the farther the image S3. is formed from the first aperture stop image, the smaller the sample cell diameter may be. f l

It will be noted that one of the principal functions of ythe lens segments G1r G13 is to produce a series of separate non-overlapping but contiguous, horizontally spaced images of the entrance slit. S0 at.V the second image Slicer R3. Even though in the specic embodiment of the invention a pluralityv of. cylindrical. lenses G1 having .vertical axes areemployed forthis purpose, other means for deflecting the imagesof the .entrance slit Sohorizontally to spread. them uniformly over. a'square area Sc. at the second image Slicer R3 may be employed. Likewise,

lit will be noted that one of the principal functions of the lens segments H1 H13V is to produceV -a series-of separate, verticallyv spacedA images A1 of lthe areaV A3 cylindrical-lenses having horizontalV axes are employed for this purpose, other means for dellect'ing the images of the area A3 vertically to spread them uniformly over a vertically elongated rectangular area and to illuminate that area substantially uniformly may be employed. In any event, it is to be noted that the first slicer R1 in effect divides an image of the square aperture stop AO into a plurality of vertically spaced, horizontally extending zones or strips, and that it deects various rays horizontally so as to form a plurality of coextensive square images of these strips at the aperture stop image A3. Likewise, it is to be noted that the second slicer R3 in eifect divides an limage of the square aperture stop image S3 into a plurality of horizontally spaced, vertically extending zones or strips, and that it dellects various rays vertically so as to form a plurality o'f coextensive images of these strips at the entrance slit SO. I

The invention has been described above in terms of an arrangement in which both horizontal `and vertical images of the aperture stop are formed at the first image Slicer R1, and both horizontal and vertical images of the entrance slit S0 are formed at the second image slicer R3. However, other arrangements may also' be employed in which radiation transmitted by reverse paths through the aperture stop A0 is divided into a number of horizontally-extending vertically-spaced slices and the same radiation transmitted along direct paths extending through the stop image S3 is divided into a plurality of vertically extending, horizontally-spaced slices. The radiation is transmitted along paths that pass thro'ugh corresponding horizontal and vertical slices or zones, but not along paths that would pass through non-corresponding Vslices or zones.

In the specific embodiment of the invention described above, it has been tacitly assumed that all of the rays entering o'ne face of a `lens segment of either image slicer also emerges from the other face of that lens segment. This condition is not necessarily true, especially if the image is narrow or the inclinations of the rays are large. Because of the fact that the various images of the entrance slit Soformed at the seco'nd Slicer are very narrow, substantial losses of energy could occur in the second Slicer if the width of the lens segments were made the same as the widths of the slit images located there. For this reason, and in order to reduce such losses, in practice lens segments having a width of 6.0 mm. have been ernployed when the width of the entrance slit images was 5.1 mm. and the horizontal displacements ko'f the slicer elements G1 were correspondingly increased.

In the forms of image slicer described above, it has been. assumed that the optic axes of successive segments of each image slicer are displaced in the same direction from the prior lens segment. It will be understood that this is not necessary, but that satisfactory results can be obtained as long as the optic axes of the various lens segments have the positions indicated above eventhough the segments of each slicer are arranged in a different order and so `long as proper correspondence exists between the lens segments of the two image slicers.

Though the invention has been described above in a form which employs lens segments to perform the image slicing and deection, it will be understood that other means may be employed to accomplish this result. For example, the two sets of lens segments of the respective image slicers may be replaced by prisms or even by mirrors. In any case, one slicer is employed for slicing an image of the aperture stop AO and the other slicer is employed for slicing an image of the entrance slit imageS3. It is to be noted that, when thus considered, the Lagrange-constant transformer of this invention slices the aperture stop image S3 when considering the direct or actual travel direction of the rays, but slices an aperture stop image Ao when considering reverse travel directio'n of rays. Such slicing of images of both aperture stops characterizes the Lagrange-constant transformer of this invention.

, trance slits S' and S0.

lten slicer elements G1', G2'

APPLICATION TO MULTIPLE-SLIT MONOCHROMATOR An embodiment of the invention employing a multipleslit spectrophotometer is illustrated in Figs. 8 and 9. This multiple-slit spectrophotometer is substantially identical with that previously described except as pointed out specifically hereinafter.

The monochromator 40' of this spectrophotometer is substantially identical with that previously described except that each of the slits S0, and SE is replaced by a double or twin slit that is a pair of closely spaced slits. Thus the entrance slit SO is replaced by a pair of en- Likewise, the intermediate slit S1 is replaced by a pair of intermediate slits S1 and S1".

Likewise, the exit slit S111 is replaced by a pair of exit slits SE' and SE".

The slits are so arranged that the first set of slits S0', S1', and SE form, in effect, one double monochromator, and the second set of slits SO, S1, and SE form a second double monochromator. However, both of these monochromators employ the same lenses, mirrors and gratings in common. The properties of multiple slit monochromators have been described by Dr. W. A. Shurcliif in the Journal of the Optical Society Am. 39,

The width of each of the entrance slits SO and S0" is 0.5 mm. and their heights are l0 cm. and their centers are spaced mm. apart. This twin-split monochromator A40 possesses the resolving power of a single-slit mono- '20. But by employing two slits of half-height, the desired transformation can be achieved by employing only ten elements in the first image slicer R1' and twenty elements in the second image slicer R2 and by employing an anamorphotic ratio of l0.

More particularly, the first image slicer R1 comprises G10' in the form of lens segments having the same focal properties as the rlens segments of the corresponding image slicer R1 of vheight as that formed in the single-slit monochromator previously described, but only half the width. The aspect ratio of each composite image Sc' and Sc" correspond- .ing'to each slit So and S" has a low aspect ratio of two, while the aspect ratio of the entire composite image formed by them has a low aspect ratio of unity.

However, it will be noted that as in the previous case -a single anamorphosed image of the virtual aperture stop AO is formed at the first image slicer R1', and that this Vimage A is sliced horizontally into ten equal square segments A1', A2' A10 by the horizontal image slicer, all as illustrated in Fig. 9. Each of these square segments yis 6.42 mm. on the side, the same as in the foregoing single-slit spectrometer, since their areas are substantially unaffected by the fact that narrower twin slits are employed.

Each of the lens segments G1', G3 G10 forms a pair of real images of the twin entrance slit SO', S0 at the second image slicer. The twenty images formed there are arranged on contiguous, side-by-side relationship,

ltwo slits S0' and S0, respectively.

thereby forming a square composite image 5l mm. on each side. Each of the individual images of each entrance slit S0 and So has a width of 2.55 mm. and a height of 51 mm.

The structure of the composite image SC thus formed at the second image slicer R2 is indicated in Fig. 10. There it will be noted that the square image is divided into twenty equal, contiguous, vertically extending, rectangular segmental images arranged in a horizontal array.

VThe centers of these images are 2.55 mm. apart. Reading from left to right when facing this image in the direction extending toward the sample, the images are arranged in two groups, namely, the iirst group S1' S0', the second group S1" S10". Each of the lens rsegments G1 thus forms two images S1 and S1" that are spaced 25.5 mm. apart horizontally, thus forming the .composite square image desired. Here, as before, the 'subscript i can have any value from 1 to 10. For example, the lens segment G1 forms the images S1' and S1" of the two slits S0' and SO" respectively. Likewise,

the lens segment G2' forms the images S2 and S2" of the And so on.

The second image slicer R2 comprises two sets of lens segments. Each of these lens segments has a width of 2.6 mm., and its face is placed directly opposite one of the segments of the composite image SC. The first group of vertical slicer elements H1 H10' are located at the positions of the images S1 S10', respectively. Thus, for example, the lens segmentv H1 is located at the position of the segmental image S1. Similarly, the lens `segment H2' is located at the position of the segmental velements H1" image S2', and so on. The second group of vertical slicer .l H10 are located at the positions of the images S1 S10", respectively. Thus, for example, the lens segments H1 is located at the position of the segmental image S1.v Similarly, the lens segment H2" is located at the position of the segmental image S1. And so on.

The lens segments H1 and H1" that are opposite images S1' and S1" formed by each segment G1 of the rst image slicer are coaxial. The lens segments of each group of slicer elements of the second image slicer R2 are arranged in echelon fashion. The optic axes of successive lens segments of each group are displaced from each other vertically by 1.94 mm.; that is, by the same amount as in the single-slit spectrophotometer. Thus, for example, the lens segments H1 and H1 are coaxial, and the lens segments H2 and H2" are coaxial, but the lens centers or optic axes of the two lens segments H2 and H3" lie in a horizontal plane 1.94 mm. beneath the horizontal plane in which the centers or optic axes of the lens segments H1 and H1'l lie.

The lenses L5 and L6 have the same properties as those of the single-slit spectrophotometer described hereinabove.

With this arrangement, the two sets of slicers cooperate to deline a beam space in the sample region which is characterized by equal vertical and horizontal Lagrange constants. More particularly, the various segments 'of the second image slicer A2 focus all the square segments of the image A' at the aperture stop image A3, the images formed there being of equal area and coextensive. Furthermore, the square composite image SC' is imaged at the aperture stop image S3.

It is thus seen that by employing image slicers, it is kpossible to alter the shape of a beam having a high 'maximum Lagrange-constant ratio in a multiple-slit monochromator to a beam having a unit maximum Lagrangeconstant ratio in a sample region. Furthermore, by applying the principles explained hereinabove, maximum Lagrange constant ratios may be transformed to provide predetermined unequal maximum Lagrange-constants in the sample region.

Though the invention has been described only with I particular reference to its application to a two-slit monochromator, it is clear that it may be also applied to @risanare nionocliromators employing `a greater number Yof slits at CELL VOPTICS In accordance with this invention, both Raman radiation and 'other `scattered radiation lare transmitted to a double monochromator. In the embodiment of the invention which makes possible the useof a minimal sample of volume, some of the Raman radiation yis internally reflected at least several times from the side walls of the sample cell before being transmitted to the monochromator. Furthermore, 'the Window or aperture stop yimage A3 through which the Raman radiation is 'transmitted from the sample cell to the monochromator is .not necessarily inscribed within the circular area formed bythe inner surface of the cell wall. As a result, background radiation is transmitted to the monochromator together with the 'Raman radiation of the sample. At the same time, the optical characteristics of the sample 'cell "14 "are so arranged that the extreme vangles ofthe rays jdeiined by the maximum Lagrange constant'ratios applicable vto the sample region lie within the angles within which total reection `from the side Walls of the sample `cell occurs. g

The use vof such reflected Raman 'radiation is not free -of disadvantages. However, these disadvantages are 'largely overcome by use of a double lmonochroniato'r. The chief advantage of employing a'double monochromator resides in its :ability to 'eliminate the elfects of radiation that is scattered by imperfections yon the `surface of or within the Wall of the sample cell. Since such radiation which originates in scattering, vlargely by the Tyndall effect, "is unchanged in `frequency, it constitutes unwanted :monochromatic radiation vhaving `the same wavelength as the radiation that `is exciting the :Raman .spectrum and which lis therefore rejected yhy .the double lmonochromator. The optical :characteristics "of Vthe u'seful beam emerging from the sample cell 14 .'-can .be aunderstood by referring to Figs. '11andl2al`ong with Figs. 3 and 4b.

Referring to Fig. 11, it is to be notedithat'the sample cell 14 is in the form of a thin-walled capillary 'tube 'that is closed at one end by means "of a iiat disk 15 perpendicular to the optic 'axis XX passing conc'enl ytrically through the Icenter of the tube. Sample cel-ls actually employed to analyze small Vsamples in this Raman 'spectrophotometer had outside diameters vof 3.0 mm. and a wall thickness of 0.1 mm., thus nproviding an internal circular cylindrical sample space that circumscribes a square cross-section of 2 mm. on each side. -A small sample T of liquid having a volume of about 0.25 ml. tills a part of such sample cell having -a length of about 80 mm. As indicated in Fig. ,11, the free end E of the liquid T may be formed within a part of the sample cell which bears an opaque coating 17, such -as black paint, at the open end of the sample cell. Even if such a coating is not employed, Raman -spectra may vbe analyzed. However, where only a predetermined'volume of sample is to be exposed to excitingradiation, as where quantitative analysis of the sample is desired, such a coating is employed to limit the portion of the sample cell that is exposed to exciting radiation to a predetermined length.

The sample cell 14 is mounted within the housing 18 with its closed end adjacent the lens L3 so that the square aperture stop images A3 and S3 of the aperture -stop A and the entrance slit S0 lie near the closed end. To explain certain optical principles, it is assumed in Fig. 11 'that the twostop limages A3 and S3 are of the same size and that they are arranged within the'liquid Tand that the area A-is near the plate l5. In practice though the diagonal of the aperture fstop'image may exceed the inside diameter of the tubular sample cell, though it is preferably less than the outside diameter of the cell.

.Exciting radiation emitted from the source l2 enters Vthe sample cell through vthe transparent walls thereof land strikes molecules m of the rliquid sample. These molecules absorb photons of exciting radiation and refradiate them with a quantized Change in energy level, scatteringthem in all directions. ln this way, the monochromatic exciting radiation yields a plurality of Raman spectrum lines vthat are characteristic of the molecules of the sample. The only radiation which can be transmitted through the monochromator 40 is radiation which is transmitted along paths, -such as the path p, which extend through-the areas of `both images A3 and S3.

For example, Raman radiation scattered from molecules m1 in the liquid that are located between the two images A3 and VS3 can travel in the direction of the path p through the image area A3 and thence through the Lagrange-constant transformer and through the monochromatorffit; .to the photocell 70. lIt Will be noted, how ever, Vthat radiation kcan be transmitted through the Lagrange-constant transformer 30 and the monochromator 40 along the same Aoptical path `even though it is radiated from molecules m3 which are beyond both of the images A3 Yand S3. Furthermore, it will be noted that radiation from :many such molecules travels along that path after being retiected at the wall or the sample vtube i4.

ln :accordance with this invention, the angle of the extreme rays which can pass through the aperture stop image =A3 vand through vthe monochromator is so established 'that the only radiation that is transmitted along .'reected paths lare those which are subject to total reflec- 1 cos -n The extreme ray inclined at the greatest angle with respect to the optic axis `X-X is a ray that travels from the end of a diagonal through one of the aperture stop vin'iages and through the end of the parallel diagonal of the other aperture sto-p image. Thus, for example, as indicated in Fig. 12, an extreme ray Pe travels from the upper rightehand corner of Yaperture stop S3 to the lower left-hand Acorner of 'aperture stop A3, as indicated in Fig. 12. In the case of the spectrophotometers described above, Athe value of the angle a for a Water sample is u=128. lt is well known that the index of refraction of almost all liquids is greater than the index of refraction of water. Thus, for example, the index of refraction of most hydrocarbon liquids is about 1.5() or more, while the index of refraction of water is about 1.33. It can be shown that if the foregoing equation for a is satisfied for water, then it is also satisfied for all liquids that have an index of refraction greater than that of water. For water, we have Since they angle of inclination a of the extreme ray is substantially less than 41 for water, it is clear that all radiation transmitted from the sample cell to the mond chromator is that which emerges from the sample cell 'along the axis X--X within va cone within which total reection occurs in the sample cell. This being true for water, it is also Vtrue for'all liquidsfhaving an index of refraction greater than that of water.

the distance between Ythe aperture stops A3 and S3, and the index of refraction.

It can be shown that the useful cross-sectional area of the sample'through which radiation is transmitted into the monochromator 40 decreases as the index Yof refraction of the liquid increases. It is thus seen that the larger the index of refraction of the sample, the smaller the diameter ofthe capillary tube may be while still taking full advantage ofthe transmissivity, or, in other words, the aperture stops of the monochromator 40.

It can be shown further that if the area of the aperture stop image S3 is made still larger without substantial change of the area of the aperture stop image A3, the angle of inclination of the extreme ray is further decreased. Such alteration may be achieved ina number of ways, such as by replacing the flat plate at the end of the sample cell 14 by a negative lens 15', as illustrated in Fig. 13. In fact, with this arrangement, the location of the aperture stop image may be moved to infinity. For this condition, the value of the angle of inclination for a given size of aperture stop A3 is reduced by a factor of about 2. For this reason, the size of the aperture stop A3 may be reduced for a given angle a, thus permitting use of even a smaller diameter sample cell.

By restricting the a'ngle of the rays to angles within which total reflection occurs, no radiation transmitted from an external Ysource such as the source 12 and through the walls 14 of the sample cell can be transmitted through the monochromator unless the radiation has been scattered, either by the molecules of the sample or by the molecules of the glass wall.

Generally speaking, the intensity of the Raman spectrum received by the monochromator is proportional to the length of the cell, that is,-to the length of the column of liquid that is exposed lto exciting radiation. Heretofore, it has been necessary to limit the length of the cell to a region in which no reflection of rays from the walls of the sample cell occurs. In accordance with this invention, the length of the sample cell is made large enough to permit at-least several internal reections. Since, so long as the Lagrange relations are satisfied, the intensity of the spectrum depends upon the length of the sample and not upon its cross-sectional area, this invention makes it possible'to produce more intense Raman spectra with small samples than was heretofore possible.

Although the invention has been described with reference to its application to a system in which Raman radiation emerges from the closed endof a sample tube, it will be understood-that the sample tube may be reversed end-to-end, and that use may be made of the radiation emerging from the open end of the tube through the meniscus of the liquid. In fact, the use of Such a meniscus may be advantageous because the meniscus acts as a negative lens.

It will be-understood, of course, that for rays inclined only slightly relative to the optic axis X- X total reflection may occur at the inner surface of the wall of the sample tube. Rays emerging from the `sample cell at such small angles are relatively free of any scattered radiation other than Raman radiation. But by employing an double monochromator in accordance with this invention, it becomes possible to collect and analyze in the monochromator radiation that emerges from the sample even over much larger angles up to the angleof total reection at the external surface of the sample cell.

For the most effective use of the invention, the length of the sample exposed to exciting radiation is Vrnade many times, and at least several times, the axial length (l) of the cell along which an extreme ray of the beam travels be- .'24 tween' successive reflections of opposite walls. words, the lengthl of the sample is such that In other where'd=,diameter of the cell and L=Lv=Ln.

It is thus seen that a Raman spectrophotometer of i greater sensitivity is provided in accordance with this in- .high sensitivity is obtained in the recording of Raman spectra of very small liquid samples by making use of multiple-reiiected radiation that is scattered from points vin the sample over a length of sample that is large. By

altering' the focal length of the lens L5 and its separation from lens L5 and the position of the sample cell, the areas of the images A3 and S3 and their distances apart may be increased, thus adapting the system for use with a large sample. Such anY arrangement is particularly useful where it is necessary to avoid the transmission of any radiation from the wall of the sample cell to the monochromator because the Raman spectrum is extremely feeble but the sample available is large. In such a case, it is desirable to employ a sample cell of convventional shapehaving a coated horn at the open end.

But even in this case, too, the employment of a Lagrangeconstant transformer is advantageous.

THE FIELD FLATTENER The mode of operation of the tield tlattener lenses L0 and LE may be understood by means of the following explanation taken in connection with the schematic diagram of Fig. 1l'.

In this diagram, a vertical section of various parts -taken 'along the optic axis X-X of the monochromator are illustrated, the omission of other parts being indicated by the dotted portions of the axis X-X. One line represents the vertical'section of the entrance slit S0. Another -line represents a vertical section of the intermediate slit SI, and the remaining line represents a vertical section of the exit slit SE. Vertical cross-sections of the two field atteuer lenses L0 and LE are illustrated. if the field flattener lens L0 were not employed, the entrance slit S0 would be imaged along the curved line IO by the action ofthe collimating mirror 48 as is well known. Likewise, if the lield flattener lens LE were not employed, the entrance slit SE Would be imaged along the curved line IE bythe action of the collimating mirror 54'. If the outeiz ends of the two images I0 and IE are located at the outer ends of the intermediate slit Si the remaining parts of the images I0 and IE are bowed away from the corresponding slits in each case being focused on the opposite side of the intermediate entrance SI from the respective terminal slits S0 and SE as indicated in the figure in highly exaggerated form.

The two eld attener lenses L0 and LE are inthe form of plano-convex lenses with their plane surfaces adjacent the slits S0 and SEV and withtheir curved surfaces facing the intermediate slit SI. These lenses L0 and LI introduceV differential retardation of the transmission of light in such a Wai;l as to remove the bow from the two images I0 and IE, thereby producing two substantially flat images coinciding with the intermediate slit SI. In this dence of the intermediate slit S1. As a result, an imaudace Aprot/ement occurs inlthe:truansmissich.r ofenergyfronjrthcY entrance slit SO to the exit slitSE.

In the particular case wherel the concave spherical mirrors Eil and 5d have focal lengths of 100 cm., tWo eld flattening. lenses L and LE which were found to be satisfactory were made of glasshavingl an index of refraction of 1.46 and the lenses radii of curvature of 11.1.3?! and vertical heights paralleltorthe slits SOV and SE of 4.5,.

The use of such eld flattener lenses increases the resolution of the monochromator byV virtue of the irnproved focussing obtained. It is to be noted that the ield flattener lenses L0 and LE need not belocatedat the terminal slits S0 and SE, but that they may be located at other points along the optic axis such as at positions adjacent to and on opposite sides of theintermediate slit S1.

SPECTRAL CHARACTERLSTICS.

As previously indicated, a Raman spectrum recorded over a range of less than one octave corresponds to yinfrared absorption and emission spectra measured l over seyeral octaves. in the Raman spectrophotometer described herein the span of wavelengths to be detected is actually confined to. a region` between about 3,500 A. tol'aboiit 6,500 A. This limitation of the span of wavelengths-to which the spectrophotometer respondsis established partly bythe characteristics of the glass employed;alongr the optical path and especially the glass used inthe Lagrangeconstant transformer Siti, andpartly by the characteristics ofthe phototube 70. Thus, when ordinary -glassis employed in the Lagrange-constant transformer, radiation having a wavelength less than about 3,500 A. is absorbed. When a phototube such as a type lPZlp'h'ototuberis employe Ito detect the radiation transmitted throughthe monochrmator, radiation havingfa wavelength greater than about 6,5 00 A. is notdetected because the sensitivity ofA the phototubeis yyery low abovethatl wavelength? I OPERATION In using the Raman spectrophotometer o-fwthisinvention, a liquid-sample is placed in the sample cell 14-and this cellis mounted in the source unit 10. rlhe mercury lamp 12 and the fan 2lare then energized in theusual' way. When the emission from the mercuryflamp has been sufliciently stabilized, the motor of the control'unit 60is energized'causing a strip of recording papento be driven past a recording element in the recorder gliv in the conventional manner and simultaneously causing the two dilraction gratings 50v and'SSfto rotate about'their axes at the same rate` thus causing radiation from successive lines of the Raman spectrum'to emerge from the exitY slit SE. As the Raman' spectrum is scanned, the intensity of the energy in the spectrum at different wavelengths is detected bythe phototube 7), amplified by the amplifier 80, andrecorded'by the recorder 90.

Part ofl a typical Raman spectrogram thus produced is` illustrated in Fig. 2. YThe intensity ofv the"various Raman lines are indicatedA by ordinates of this graph. The differential frequencies are indicatedv by the' abscis'sa. However, the peak indicated' atV zero abscissa does n ot represent theV intensity of the Rayleighline of the same Wavelength as the exciting` radiationfbec'ause its intensity is too great to be displayedl simultaneously together with` the other, weaker peaks. The other peaks indicate the intensity of Ramanflines corresponding to various permitted changes ofjenergy leyelfoff` the sample underysoins analysis- Itis, to be Maltha? a Certain amoumgf'backaround and stray radiation appears onv the spectrograrn as indicated by the lower portions ofthe graph between the peaks. The eiiects of such background radiation including thatwhich isY scattered from the wallsofthe-` sample tube is greatly reduced 4at every Wavelength by the'use of the double monochromator, thus makingpossible liable detection of the Raman spectrum of small samples.

Though variouslfeaturesofzthisyinvention have been described with, particular referencedo their application to Raman Spectroscopy, it is Clear that some features of theY inventonmay. be employed inv many Fother. ways- Furthermore, it is .clear that, although only two pariflllarl forms of Raman spectrophotometer have been specificallyI disclosed herein, it will be obviousthat the invention is not limited thereto, but is capable of lembo diment in other. forms. Various changes which will now suggest themselves to those skilled in the art may. be made in the material, form, details of constructionY and arrangement ofthe elements without departing from theV inven-l tion.

The. invention claimed is;

l. In a spectrometer, means for supporting a sample for analysis in a sample region on an optic axis, said sample emitting radiation; a monochromator. having an entrancey slit and having a dispersing element arranged to forma spectrum of heterochrornatic radiation enter-l ing said entrance slit, said monochromator having a rst principa-lplane parallel to the length of said slit and haying a second'principal plane parallel to the width of said slit, the length. of said slit beingtlarge compared to the widthof said slit, the beam transmitted through said entrance slit and through said-dispersing element being char,- acterized by a maximum Lagrange aperture constant in said first principalplane that is large compared with the maximum Lagrange aperture constant in said second principal plane; and'imaging means for collecting radiation that emerges from said sample alongsaid optic axis and for transmitting such radiation through said entrance slit and toward said dispersing element, the beam of radiatien so transmitted from the sample being characf terized in the sample region by maximum Lagrange apenture constants in said two planes that haye a low. ratio compared with the ratio of the aforesaid Lagrange aperture constants, said imaging means comprising a Lagrange constant transformer including rst and second spacedapart crossed beam-detiecting units, each ofthe two beamdeecting units having a series of parts that correspond with a series of parts in the other beam-deecting unit, each pair of corresponding parts of the two deflecting units forming at said entrance` slit substantially coexteng sive images. by means of such radiation that emerges frornsaid sample region along paths that extend through differenty zones of said sample region, which zones exf tend in the direction of said iirst principal plane but are located at different positions alongsaid second principal plane, andeach pair of cornesponding parts of said two deiiecting` units forming within said monochromator images of a common area lyingA in said sample region, said images forming zones that extend, in the direction of said second principal plane but yare spaced apart in the directionpf` said first principal plane.

2. A spectrometer. as defined in claim l in which each of. said beam-deflecting units comprises a plurality of cylindricalY focussingA segments having axes that ylie parallel to. said optic axis, thel opticaxes of the segments of saidV iirst be-am-deecting unit being spaced apart in the direction of said rst. principal plane, the optic axes of the segments of said second beam-deectng unit `being spaced apart in the, direction of said second principal plane.

3. A spectrometer as defined in claim 2 in which the cylindrical focussing elements of each of said` beam-de,- fleetingw units are transparent and their edge faces are parallel tosaid optic axis and in which said cylindrical focussing elements of each unit are rigidly secured togethenwith the adjacent faces of successive parts lying close together.

4L A spectrometer as dened in claim l in which said imaging means comprises anamorphotic means that Vhas a magnification in said first plane that is large compared with the magnification in said second plane when considering radiation traveling from said sample region to- 27 ward said monochromator and in which the square of the ratio of said first magnification to said second magnificat-ion equals the ratio of the maximum Lagrange aperture constant ratio of said monochromator and the maximum Lagrange aperture constant ratio of said sample region.

5. In a spectrometer, means for supporting a sample for analysis in a sample region on au optic axis, said sample emitting radiation; a monochromator having an entrance slit and having a dispersing element operatively arranged to form a spectrum of heterochromatic radiation that enters said entrance slit, said monochromator having a Y of said tube, said imaging means comprising a Lagrange first principal plane parallel to the length of said slit and having a second principal plane parallel to the width of said slit, the length of said slit being large compared to the width of said slit, said slit defining one aperture stop having a very large aspect ratio, other parts of said monochromator delining a separate aperture stop of low aspect ratio, said aperture stops limiting a beam of radiation of any wavelength that is transmitted from the entrance slit to the dispersing element, said limited beam being characterized by a maximum Lagrange aperture constant in said rst principal plane that is large compared with the maximum Lagrange aperture constant in said second principal plane; and imaging means for collecting radiation that emerges from said sample along said optic axis and for transmitting such collected radiation through said entrance slit and toward said dispersing element, the beam of the radiation so transmitted from the sample emerging from a region characterized by substantially equal maximum Lagrange aperture constants in said principal planes, both of the aperture stop images defining said latter Lagrange aperture constants having small aspect ratios, said imaging means comprising a Lagrange constant transformer including first and second spacedapart crossed beam-deflecting units, each of the two beam-deecting units having a series of parts that correspond with a series of parts in the other beam-deflecting unit, each pair of corresponding parts of the two deflecting units forming at said entrance slit substantially coextensive images by means of such radiation that emerges from said sample region along paths that extend through different zones, which zones extend in the direction of said first principal plane but are located at different positions along said second principal plane, and the respective pairs of corresponding parts of said two deecting units forming within said monochromator images of a common area lying in said sample region, said images forming zones that extend in the direction of said second principal plane but are spaced apart in the direction of said iirst principal plane.

6. In a spectrometer, means for supporting an elongated tube of circular cross section for holding a sample in a sample region with the axis of the tube arranged along an optic axis; a monochromator having an entrance slit and having a dispersing element arranged to form a spectrum of heterochromatic radiation entering said entrance slit, said monochromator having a rst principal plane parallel to the length of said slit and having a second principal plane parallel to the Width of said slit, the length of said slit being large compared to the width of said slit, the beam transmitted by said entrance slit and said dispersing element being characterized by a maximum Lagrange aperture constant in said first principal plane that is large compared with the maximum Lagrange aperture constant in said second principal plane; and imaging means for collecting radiation that emerges from said sample along said optic axis and through one end of said tube and for transmitting such radiation through said entrance slit and toward said dispersing element, the angle over which such radiation is transmitted from said sample being so large and the length of the sample from which radiation emerges being so long that such transmitted radiation includes radiation emitted by a sample in such tube and multipli-reilected at the walls constant transformer including first and second spacedapart crossed beam-dciiecting units, each of the two beam-deecting units having a series of parts that correspond with a series of parts in the other beam-deflecting unit, each pair of corresponding parts of the two deflecting units forming at said entrance slit substantially coextensive images by means of such radiation that emerges from said sample region along paths that extend through different zones, which zones extend in the direction of said rst principal plane but are located at dilerent positions along said second principal plane, and the respective pairs of corresponding parts of said two deccting units forming within said monochromator images of a common area lying in said sample region, said images forming zones that extend in the direction of said second principal plane but are spaced apart in the direction of said first principal plane.

7. In a Raman spectrometer, means for supporting an elongated sample holding tube in a sample region with the axis of the tube arranged along an optic axis; a double monochromator having an entrance slit and an exit slit and an intermediate slit and comprising one monochromator section, including a first dispersing element, operatively arranged between said entrance slit and said intermediate slit, and comprising a second monochromator section, including a second dispersing element, operatively arranged between said intermediate slit and said exit slit,

said monochromator having a first principal plane parallel to the length of said slits and having a second principal plane parallel to the width of said slits, the lengths of said slits being large compared to the widths of said slits, a beam of radiation of any wavelength that is transmitted from the entrance slit to the first dispersing element and thence to the exit slit being characterized by a maximum Lagrange aperture constant in said iirst principal plane that is large compared with the maximum Lagrange aperture constant in said second principal plane; means for illuminating a sample in said sample holding tube throughout an extended length thereof with exciting radiation; imaging means for collecting radiation that emerges from said sample along said optic axis and through a substantially square area that occupies a large fraction of one end of said tube and for transmitting such radiation through said entrance slit and toward the dispersing element of said iirst monochromator section, the beam of the radiation so transmitted from the sample region emerging from a region in said sample characterized by substantially equal maximum Lagrange aperture constants in two planes corresponding to the principal planes of said monochromator, the angle over which such radiation is transmitted from said sample being so large in any plane and the length of the sample that is illuminated by exciting radiation being so long that such transmitted radiation includes radiation scattered by a sample in such tube and multipli-reflected at the walls of said tube, said imaging means comprising a Lagrange constant transformer including first and second spacedapart crossed beam-deflecting units, each of the two beamdeflecting units having a series of parts that correspond with a series of parts in the other beam-deilecting unit, each pair of corresponding parts of the two deflecting units forming at said entrance slit substantially coextensive images by means of such radiation that emerges from said sample region along paths that extend through different zones, which zones extend in the direction of said iirst principal plane but are located at different positions along said second principal plane, and the respective pairs of corresponding parts of said two detlecting units also forming within said monochromator images of said square area, said images forming zones that extend in the direction of said second principal plane but are spaced apart in the direction of said first principal plane; and means responding to the intensity of radiation emerging from said exit slit.

, .Amy

8, In a Raman spectrometer, means for-supporting an elongated sample holding tube of circular cross section in a sample region with the axis of the tube arranged along an optic axis; a double monochromator having an entrance slitand an exit slit and an intermediate slit, and comprising one monochromator section, including a first dispersing element, operatively arranged between said entranceslit and saidintermediate slit, and also comprising a secondmonochrornator section, including a second dispersing element, operatively arranged between said intermediate slit and` said exitslit, said monochromator having a first principal ,plane parallel to the lengths of said slits and having a second principal plane parallel to the widths, of said slits, the lengths ofrsaid slits being large compared to the widthsk of saidslits, one of said slits `defining one aperturestop having a very large aspect ratio, other parts of said monochromator delining a separate aperture stop of low aspect ratio, said aperture stops limiting a beam of radiation of any wavelength that is transmitted from the entrance slit to the first dispersing element and to the intermediate slit and to the second dispersing element and thenceto the exit slit, said limited beam being characterized by a maximum Lagrange aperture constant in said first principal plane that is large compared with the maximum Lagrange aperture constant inv said second principalplan'e; means for illuminating a sample in said sample holding tube throughout an extended length thereof with exciting radiation; imaging means for collecting radiation that emerges from said sample along, said;v optic axis and through a substantially square area at one end of said tube and for transmitting such radiation through said entrance slit and toward the dispersing element of said first monochromator section, the radiation so transmitted from the sample region emerging from a region in said sample characterized by substantially equal Lagrange aperture constants in two planes corresponding to the principal planes of said monochromator, said imaging means comprising a Lagrange constant transformer including anamorphosing means and also including first and second spaced-apart crossed beam-deiiecting units, each of the two beamdeecting units having a series of parts that correspond with a series of parts in the other beam-deflecting unit, each pair of corresponding parts of the two detiecting units cooperating to form at said entrance slit substantially coextensive images by means of such radiation that emerges from said sample region along paths that extend through different zones, which zones extend in the direc tion of said first principal plane but are located at difierent positions along said second principal plane, and the respective pairs of corresponding parts of said two defiecting units also forming at said second aperture stop rectangular images of said square area, said rectangular images being spaced apart in the direction of said first principal plane; and means responding to the intensity of radiation emerging from said exit slit.

9. In an optical system for transmitting a beam of radiation from a limited space in a first region through a limited space in a second region along an optic axis, means for rendering the ratio of maximum Lagrange aperture constants of said beam in two mutually perpendicular planes parallel to the optic axis in one space different from the ratio of maximum Lagrange aperture constants of said beam in corresponding planes parallel to the optic axis in the other space, and for rendering the product of said two Lagrange aperture constants in one space equal to the product of said two Lagrange aperture constants in the other space, said means including: means for focussing in a first image area an image of a crosssectional area transverse to the optic axis in the first space, said first image area comprising a first set of zones that are arranged in a vertical array; means for focussing in a second image area an image of a cross-sectional area transverse to the optic axis in the second space, said second image area comprising a second set of zones that E@ l are arranged'in a horizontal array,- the number of zones'` in the two sets being equal, one zone in each set cerresponding to one zone in the other set; and means for selectively transmitting radiation that passes through each zone in each set through the corresponding zone inthe other set.

10. ln an optical system for transmittinga beam of radiation from a limited space in a first region through a limited space in a second region, means for rendering the ratio of maximum Lagrange aperture constants in first and second mutually perpendicular planes parallel to the optic axis in one space being different from`v the ratio of maximum Lagrange aperture constants in corresponding planes parallel to the optic axis in the other space, and for rendering the product of the two Lagrange constants in one space equal to said product of the two Lagrange aperture constants in the other space, said means including; means for focussing in a first image area an image of a cross-sectional area transverse to the optic axis in the first space, said first image area comprising a first set of zones that are spaced apart vertically; means for focussing in a second image area an image of a cross-sectional area transverse to the optic axis in the second space, said second image area comprising a second set of zones that are spaced apart horizontally, the number of zones in the two sets being equal, one zone in each set corresponding to one zone in the other set; and means for selectively transmitting radiation that passes through each zone in each set through the corresponding zone in the other set, said last mentioned means comprising first deflection means located at said first image area for horizontally deliecting radiation passing through the respective horizontally extending zones by such amounts that such radiation passing through said first image area and through each horizontally extending zone also passes through the corresponding vertically extending zone, and second deiiection means located at said second image area for vertically deiiecting radiation passing through the respective vertically extending zones by such amounts that such radiation passing through said second image area and through each vertically extending zone also passes through the corresponding horizontally extending zone.

11. In an optical system as defined in claim l0 comprising a monochromator defining one of said regions, said monochromator having two spaced apart beamlimiting elements that define the limits of the beam of radiation that is transmitted both through said first region and through said second region.

l2. In a spectrometer, means for supporting a tube for holding a sample in a sample region with the axis of the tube arranged along an optic axis; a double monochromator having an entrance slit and an exit slit and an intermediate slit and comprising one monochromator section including a iirst dispersing element, operatively arranged between said entrance slit and said intermediate slit, and comprising a second monochromator section, including a seco-nd dispersing element, operatively arranged between said intermediate slit and said exit slit; imaging means for collecting radiation that emerges from sample along said optic axis and through one end of said tube and for transmitting such radiation through said entrance slit and toward said first dispersing element, the angle over which such radiation is transmitted from said sampie being so large in any plane and the length of the sample that is illuminated by exciting radiation being so long that such transmitted radiation includes radiation scattered by a sample in such tube and multiplyreected at the walls of said tube; and means responding to ti e intensity of radiation emerging from said exit slit.

13. ln a spectrometer, means for supporting a sample for analysis in a sample region on an optic axis, said sample emitting radiation; a multiple slit monochromator having a plurality of parallel entrance slits and having 

